provide pathlib based paths in compat and plain flavours to enable removal of py.path usage in pytest 3.0

[RonnyPfannschmidt]

in order to get rid of the py.path legacy it is neccessary to provide a upgrade path
i propose adding a config option to handle legacy, compat and path based path objects for tmpdir, cache & co

leacy should be the default in the 2.9 release
if there is a 2.10 it should make compat the default

3.0 should drop the legacy and only offer pathlib

the canonical implementation should be pathlib2, since its on a recent state and works on 2.7, and 3.x

the compat flavour should warn with deprecationwarning subclasses to note the method removal in pytest 3.0

[RonnyPfannschmidt]

clearing the milestone, perhaps aiming for 4.0 is more realistic